When changing the subtraction to addition of the opposite, you forgot to change $\displaystyle-{7}\usepackage{cmbright}\Huge\$ to $\displaystyle+{7}\usepackage{cmbright}\Huge\$ . You should add $\displaystyle{\left(-{6}{{g}}^{{2}}+{11}{g{-}}{3}\right)}+{\left(-{4}{g{+}}{7}\right)}\usepackage{cmbright}\Huge\$ . The correct answer is $\displaystyle-{6}{{g}}^{{2}}+{7}{g{+}}{4}\usepackage{cmbright}\Huge\$ .